1. Field of the Invention
The present invention relates to a power stage, and in particular, to a power stage for an operational amplifier.
2. Discussion of the Related Art
As is known, a wide variety of power stages exist for operational amplifiers featuring NPN power output transistors, and which are classified according to the type of biasing employed. In the following description, reference will be made to a class AB amplifier, in which, even when in a quiescent condition, the transistors are supplied with a weak current to eliminate crossover distortion.
Accurate control of the quiescent current is especially important in the case of class AB amplifiers, and is normally done using complex circuit networks including numerous NPN and PNP components. However, such complex circuit networks involve several drawbacks. For example, the control network introduces numerous poles, which impair the operational amplifier band, involves a far from negligible increase in the area required for integrating the operational amplifier, and is not always capable of accurately controlling and reproducing the quiescent current.
In addition, power stages of this type are also unstable when a capacitive load is connected to the output; this instability is eliminated by providing a network (known as a Boucherot cell) comprising the series connection of a resistor with a very low resistance (a few .OMEGA.), and a capacitor with a capacitance in the order of hundreds of nF and located between the output of the operational amplifier and ground. Such a cell, however, presents the drawback of increasing the cost of the amplifier.